It comes almost six months to the day after a 6.5 magnitude earthquake, centred near the Marlborough town of Seddon, set off a swarm of tremors that rocked the middle of the country for months.

The shaking from yesterday's quake was felt from Invercargill to Auckland but those in Manawatu, Wairarapa and Wellington bore the brunt of it.

Power has been returned to all customers. Both commuter rail and freight rail services around the capital had returned to normal this morning after they were suspended yesterday. All state highways are open this morning but police advise drivers to take care.

GNS duty seismologist Caroline Little said the quake was caused by the Pacific tectonic plate ''subducting'' under the Australian plate.

The quake could not be pinned on an individual faultline and was quite different from two historic quakes in the area that caused serious damage.

It was expected the next seven days would see another quake – or up to five – of between magnitude 5 - 5.9.

A much larger quake could not be ruled out but was "unlikely", Little said.

THE DAMAGE

There were no reports of death or serious injury to people, but numerous reports of damage to property.

An Earthquake Commission spokeswoman said figures for the numbers of claims lodged so far would not be available for a few days.

However, it planned to later today release a final date for lodging claims.

A massive chunk of 162-metre-high Castle Rock, at Castlepoint, was shaken loose.

It was believed the boulder weighed about 20 tonnes and was the size of a small car. It sheared off a limestone face, south of The Gap beach, and bounced down a steep slope towards the beach.

A large slip near Mangaweka also turned the Rangitikei River milky-grey.

There were further reports of slips along the Pohangina River and at Anzac Cliffs on the Manawatu River.

Electricity was cut to about 5600 homes in Tararua, Manawatu and Taranaki but but lines company Powerco confirmed all were back on this morning.

Wairarapa Civil Defence emergency management controller Kevin Tunnell said there had been numerous reports of minor damage at homes across Masterton and the wider Wairarapa.

The quake was felt so strongly that Masterton Deputy Mayor Graham McClymont, who was playing golf in Eketahuna when it struck, feared Masterton would be "lying in a pile of rubble".

Near Masterton, Mauriceville residents John Hart and Karen Monks said their restored villa would need a second restoration to undo quake damage.

"The house is pretty much trashed. There's crockery everywhere, the fridge has emptied itself, we've got appliances on the floor, bookcases down, and there's quite a few cracks around the house."

The Insurance Council of New Zealand said it did not know the estimated cost of quake damage from yesterday's shake.

The value of damages was not usually known until a "few weeks" after an event like an earthquake, a spokesperson said, and no information had been received yet about any claims.

Large cracks also forced the closure of the main road to the tiny Tararua town of Pongaroa.

A structural engineer would carry out a further examination on the three-storey residential Daniels Building in Masterton after a dangerous building declaration was issued yesterday, Tunnell said.

In Manawatu, there were reports of chimneys collapsing, cracks appearing in walls, and television sets falling over. A Palmerston North resident said concrete had sunk around their home.

Leeds St resident Maria Dalziel said her brick fence toppled "like a house of cards" when the shaking started. Inside, her collection of 100 antique glass bottles had all smashed.

Melanie Iosefa was in the central Palmerston North NZI insurance building when plaster fell on to a desk. The 15 to 20 workers in the building were forced to duck for cover on the floor but one woman sitting underneath received a blow to the head as she took cover under a table.

Milk was being sent to other plants until it was considered safe to resume operations there.

This was a normal procedure and would not affect the wider supply chain, the company said.

The quake-prone Farmers building in Wellington's Cuba Mall was evacuated while inspectors checked for damage.

In Lower Hutt's Queensgate Mall, shoppers were left screaming and ducking for cover as items fell off shelves. Some stores closed afterwards.

Supermarkets in the Wellington region sustained only minor damage.

KiwiRail spokeswoman Sophie Lee said there were no reports of any damage to the rail network or Wellington Railway Station.

PM REACTS

Prime Minister John Key said people should be prepared for aftershocks.

"There have been minor aftershocks and that's to be expected. I guess one of the big lessons that came out of Christchurch for those who don't spend our lives studying these things, is that when you get a big jolt you are going to get a series of aftershocks that go on for a long period of time," he said.

"This was a very large earthquake in terms of the Richter scale, consistent with what we saw in Christchurch, it's just that it was in a deeper area.

"Certainly a big jolt, and unnerving for the people of the Wairarapa area and the wider Wellington area.

"Mainly what we can see is surface damage, stuff off shelves, some minor damage to chimneys and lights but people will be keeping a watching eye on it over the next few days and weeks."

TSUNAMI WARNING

Late last year, GNS Science released a report showing a tsunami up to 35 metres high could cause as much devastation over Wellington and Napier as the 2011 waves caused in Japan.

GNS duty seismologist Caroline Little confirmed the ''worst case scenario'' would be triggered by the same two plates that caused yesterday's quake. However, unlike yesterday, when the epicentre was deep inside the Pacific plate, for a tsunami to be triggered the plates would have to clash at the ''interface'' of the two and be centred out to sea.

A larger quake would also be needed to trigger a tsunami, she said. The report, by natural hazard scientist Graham Leonard, said waves could reach 15 metres above normal sea level in Wairarapa, Northland, Great Barrier Island, and parts of East Cape. But Wellington's steep valleys could amplify tsunami waves up to 35m, he said.

''No part of the New Zealand coastline is free from tsunami hazard,'' it warns.

Little said yesterday's quake was different from two historic quakes in the area that caused serious damage.

HISTORIC QUAKES

A magnitude 8.2 quake on the Wairarapa Fault in 1855 remains the most violent in New Zealand since European settlement.

A 10-metre tsunami struck Palliser Bay on the south Wairarapa coast. The tsunami also flooded Porirua Harbour, and hit Titahi Bay and the Kapiti Coast. In Wellington, a four to five metre tsunami swept over the isthmus between Lyall Bay and Evans Bay.

In June, 1942, a 7.2 quake centred near Masterton shifted houses on their foundations and caused some roofs to collapse.

In Masterton, damage was widespread and only few buildings survived unscathed.

In Wellington, 100 city buildings were found to be seriously damaged, about 10,000 chimneys toppled and 5000 homes were in need of extensive repairs.

Little said yesterday's quake was more similar to a 6.4 quake centred near Weber, Tararua in May, 1990. That one caused no major damage.

Correction: This story earlier incorrectly reported that seals were killed in a rock collapse at Castle Point.